Method and computer code for statistical process control for censored production data

ABSTRACT

A method for monitoring device characteristics of semiconductor integrated circuits. The device characteristics includes censored data and uncensored data. The method includes determining a plurality of minimum breakdown voltages numbered from 1 through N, respectively, for a plurality of lots (e.g., wafer fabrication lots) numbered from 1 through N. Each of the plurality of minimum breakdown voltages is respectively indicative of the plurality of samples through order statistics. One or more of the plurality of samples includes one or more uncensored data points and one or more censored data points. The method includes processing the minimum breakdown voltages, respectively, for the plurality of lots. Each of the minimum breakdown voltages is processed for the respective plurality of lots and is indicative of a population characteristic breakdown voltage numbered from 1 through N for the respective lot numbered from 1 through N. The method includes determining one or more anomalies based upon the processing of the minimum breakdown voltages. The one or more anomalies is associated with one or more processes associated with at least one of the lots.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Application No.200810034008.4, filed on Feb. 25, 2008, commonly assigned, and of whichis hereby incorporated by reference for all purposes.

COPYRIGHT NOTICE

Certain portions of the present specification include computer codes,where notice is hereby given. All rights have been reserved underCopyright for such computer codes, by © 2004 and 2005 SemiconductorManufacturing International (Shanghai) Corporation, which is the presentassignee.

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits and theirprocessing for the manufacture of semiconductor devices. In particular,the invention provides a method and system for monitoring andcontrolling process related information for the manufacture ofsemiconductor integrated circuit devices. More particularly, theinvention provides a method and system for monitoring devicecharacteristics, including censored data and uncensored data, ofsemiconductor integrated circuits in the manufacture of semiconductorintegrated circuit devices. But it would be recognized that theinvention has a much broader range of applicability.

Integrated circuits have evolved from a handful of interconnecteddevices fabricated on a single chip of silicon to millions of devices.Conventional integrated circuits provide performance and complexity farbeyond what was originally imagined. In order to achieve improvements incomplexity and circuit density (i.e., the number of devices capable ofbeing packed onto a given chip area), the size of the smallest devicefeature, also known as the device “geometry”, has become smaller witheach generation of integrated circuits.

Increasing circuit density has not only improved the complexity andperformance of integrated circuits but has also provided lower costparts to the consumer. An integrated circuit or chip fabricationfacility can cost hundreds of millions, or even billions, of U.S.dollars. Each fabrication facility will have a certain throughput ofwafers, and each wafer will have a certain number of integrated circuitson it. Therefore, by making the individual devices of an integratedcircuit smaller, more devices may be fabricated on each wafer, thusincreasing the output of the fabrication facility. Making devicessmaller is very challenging, as each process used in integratedfabrication has a limit. That is to say, a given process typically onlyworks down to a certain feature size, and then either the process or thedevice layout needs to be changed. Additionally, as devices requirefaster and faster designs, process limitations exist with certainconventional processes, including monitoring techniques, materials, andeven testing techniques.

An example of such processes includes ways of monitoring process relatedfunctions during the manufacture of integrated circuits, commonly calledsemiconductor devices. Such monitoring process is often desired forcontinuously improving quality and productivity to stay competitive. Asmerely an example, statistical process control (SPC) has been playing animportant role in conventional industries. It is a procedure in whichdata are collected, organized, analyzed and interpreted. Actions arerequested to identify root causes and to implement solutions so aprocess can be maintained at its desired level or be improved to ahigher level. SPC makes use of statistical signals to identify sourcesof variation, to correct identified variation causes therefore toimprove performance, and to maintain control of processes.

Conventional SPC control limits determination often assumes a normaldistribution and no censored data is provided in the data collection. Inmany cases in reliability data collection data censoring occursfrequently. For example, the voltage breakdown (VBD) data of inter-layerdielectric “ILD” measured with ramped up voltage could be censored whensome of dice of the sample do not have breakdown failure at a maximumramped up voltage. In these cases, a direct application of an X-Bar andS control chart on the observed data is often not adequate. Accordingly,data collection is not reliable.

From the above, it is seen that an improved technique for manufacturingsemiconductor devices is desired.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, techniques directed to integratedcircuits and their processing for the manufacture of semiconductordevices are provided. In particular, the invention provides a method andsystem for monitoring and controlling process related information forthe manufacture of semiconductor integrated circuit devices. Moreparticularly, the invention provides a method and system for monitoringdevice characteristics, including censored data and uncensored data, ofsemiconductor integrated circuits in the manufacture of semiconductorintegrated circuit devices. But it would be recognized that theinvention has a much broader range of applicability.

In a specific embodiment, the present invention provides a method formonitoring device characteristics of semiconductor integrated circuits.The device characteristics includes censored data and uncensored data.The method includes determining a plurality of minimum breakdownvoltages numbered from 1 through N, respectively, for a plurality oflots (e.g., wafer fabrication lots) numbered from 1 through N. Each ofthe plurality of minimum breakdown voltages is respectively indicativeof the plurality of samples through order statistics. In a preferredembodiment, the lot corresponds to a plurality of semiconductor wafersbeing processed through one or more processes for the manufacture ofintegrated circuits or other devices. One or more of the plurality ofsamples includes one or more uncensored data points and one or morecensored data points. The method includes processing the minimumbreakdown voltages, respectively, for the plurality of lots. Each of theminimum breakdown voltages is processed for the respective plurality oflots and is indicative of a population characteristic breakdown voltagenumbered from 1 through N for the respective lot numbered from 1 throughN. The method includes determining one or more anomalies based upon theprocessing of the minimum breakdown voltages. The one or more anomaliesis associated with one or more processes associated with at least one ofthe lots. The method provides an output based upon the one or moreanomaly.

Many benefits are achieved by way of the present invention overconventional techniques. For example, the present technique provides aneasy to use process that relies upon conventional technology. In someembodiments, the method provides higher device reliability andperformance. Depending upon the embodiment, one or more of thesebenefits may be achieved. These and other benefits will be described inmore throughout the present specification and more particularly below.

Various additional objects, features and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified flow chart illustrating a method according to anembodiment of the present invention;

FIG. 2 is a simplified diagram illustrating a general purpose computerthat is capable of performing the method illustrated in FIG. 1 accordingto an embodiment of the present invention;

FIG. 3 is a simplified plot of breakdown voltage according to anembodiment of the present invention;

FIG. 4 is a simplified plot of transformed data for breakdown voltageaccording to an embodiment of the present invention;

FIG. 5 is a histogram plot of transformed data according to anembodiment of the present invention;

FIG. 6 is a control chart according to an embodiment of the presentinvention;

FIG. 7 is a simplified computer system according to an embodiment of thepresent invention;

FIG. 8 is a simplified block diagram of a computer system according toan embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, techniques directed to integratedcircuits and their processing for the manufacture of semiconductordevices are provided. In particular, the invention provides a method andsystem for monitoring and controlling process related information forthe manufacture of semiconductor integrated circuit devices. Moreparticularly, the invention provides a method and system for monitoringdevice characteristics, including censored data and uncensored data, ofsemiconductor integrated circuits in the manufacture of semiconductorintegrated circuit devices. But it would be recognized that theinvention has a much broader range of applicability.

In a specific embodiment, the present invention provides a method formonitoring device characteristics of semiconductor integrated circuitsas follows: see FIG. 1.

1. Provide a plurality of lots (each of the lots having a plurality ofsemiconductor wafers being processed, e.g., complete functionalintegrated circuits or partial devices but functional elements) numberedfrom 1 through N;

2. Determine a plurality of minimum breakdown voltages numbered from 1through N, respectively, for a plurality of lots numbered from 1 throughN;

3. Using order statistics to associate the plurality of minimumbreakdown voltages to a plurality of samples (e.g., wafers);

4. Store information from at least one or more of the plurality ofsamples including one or more uncensored data points and one or morecensored data points;

5. Process the minimum breakdown voltages, respectively, for theplurality of lots (each of the minimum breakdown voltages processed forthe respective plurality of lots is indicative of a populationcharacteristic breakdown voltage numbered from 1 through N for therespective lot numbered from 1 through N);

6. Determine one or more anomalies based upon the processing of theminimum breakdown voltages (the one or more anomalies is associated withone or more processes associated with at least one of the lots;

7. Provide an output based upon the one or more anomaly;

8. Perform other steps, as desired; and

9. Stop.

The above sequence of steps provides methods according to an embodimentof the present invention. As shown, the method uses a combination ofsteps including a way of performing a SPC process according to anembodiment of the present invention. Many other methods and system arealso included. Of course, other alternatives can also be provided wheresteps are added, one or more steps are removed or repeated, or one ormore steps are provided in a different sequence without departing fromthe scope of the claims herein. Additionally, the various methods can beimplemented using a computer code or codes in software, firmware,hardware, or any combination of these. Depending upon the embodiment,there can be other variations, modifications, and alternatives. Furtherdetails of the present method and system can be found through out thepresent specification and more particularly below.

FIG. 1 is a simplified flowchart 100 illustrating a method according toan embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims herein.One of ordinary skill in the art would recognize other variations,modifications, and alternatives. FIG. 2 is a simplified diagramillustrating a general purpose computer that is capable of performingthe method illustrated by the flowchart 100 according to an embodimentof the present invention. This diagram is merely an example, whichshould not unduly limit the scope of the claims herein. One of ordinaryskill in the art would recognize other variations, modifications, andalternatives. As shown, the present method begins at start, step 101. Ina specific embodiment, the present invention provides a method formonitoring device characteristics of semiconductor integrated circuits.

As shown, the method begins by providing (step 103) a plurality of lots(each of the lots having a plurality of semiconductor wafers beingprocessed, e.g., complete functional integrated circuits or partialdevices but functional elements) numbered from 1 through N. In aspecific embodiment, each of the lots includes one or more structuresthat are capable of being characterized by a breakdown voltageassociated with an integrated circuit. In a specific embodiment, theintegrated circuit can include a memory device, application specificintegrated circuit, commonly called ASICS, or microprocessor-type (e.g.,microprocessor, digital signal processor, or microcontroller device). Ofcourse, there can be other variations, modifications, and alternatives.

In a specific embodiment, the method includes determining (step 105) aplurality of minimum breakdown voltages numbered from 1 through N,respectively, for a plurality of lots numbered from 1 through N. In aspecific embodiment, the breakdown voltages are measured using aconventional testing technique. Depending upon application, breakdownvoltages may be generated in various ways. Of course, there can be othervariations, modifications, and alternatives.

Referring back to FIG. 1, the present method includes using (step 107)order statistics to associate the plurality of minimum breakdownvoltages to a plurality of samples (e.g., wafers). The method includesstoring information (step 109) from at least one or more of theplurality of samples including one or more uncensored data points andone or more censored data points. As an example, FIG. 2 illustrates away of associating the breakdown voltages to the samples using orderstatistics. Of course, there can be other variations, modifications, andalternatives.

In a specific embodiment, the method includes processing (step 111) theminimum breakdown voltages, respectively, for the plurality of lots(each of the minimum breakdown voltages processed for the respectiveplurality of lots is indicative of a population characteristic breakdownvoltage numbered from 1 through N for the respective lot numbered from 1through N). Depending on applications, the breakdown voltage may bemeasured in various ways. Of course, there can be other variations,modifications, and alternatives.

Referring back to FIG. 1, the method includes determining one or moreanomalies based upon the processing of the minimum breakdown voltages ina specific embodiment. In a specific embodiment, the one or moreanomalies is associated with one or more processes associated with atleast one of the lots. In a specific embodiment, the one or moreprocesses can include implantation, deposition, photo masking, etching,diffusion, any combination of these, and the like. Of course, there canbe other variations, modifications, and alternatives.

In a specific embodiment, the method includes providing an output (step115) based upon the one or more anomaly. In a specific embodiment, themethod includes outputting using a plot (e.g., in graphical form), aprint out, or other output suitable for use with the present method andsystem. In a specific embodiment, the method ends at step 117, which isstop. Of course, there can be other variations, modifications, andalternatives.

The above sequence of steps provides methods according to an embodimentof the present invention. As shown, the method uses a combination ofsteps including a way of performing a SPC process according to anembodiment of the present invention. Many other methods and system arealso included. Of course, other alternatives can also be provided wheresteps are added, one or more steps are removed or repeated, or one ormore steps are provided in a different sequence without departing fromthe scope of the claims herein. Additionally, the various methods can beimplemented using a computer code or codes in software, firmware,hardware, or any combination of these. Depending upon the embodiment,there can be other variations, modifications, and alternatives.

Example: To prove the principle and operation of the present method andsystem, we performed various experiments. These experiments are merelyexamples, which should not unduly limit the scope of the claims herein.One of ordinary skill in the art would recognize many variations,modifications, and alternatives. As background information, weunderstand that conventional SPC control limits determination assumesnormal distribution and no censored data happened in the datacollection. In many cases in reliability data collection data censoringhappens frequently. For example, the VBD data of iLD measured withramped up voltage could be censored when some of dice of the sample donot have breakdown failure at a maximum ramped up voltage. In thesecase, a direct application of an X and S control chart on the observeddata is not adequate anymore.

There exist a method called “CEV” control limit calculation inliterature (published by Stefan H. Steiner etc, in J. of QualityTechnology, Vo. 32, No. 3, pp. 199-208, 2000) for censored observations.They use X ban and Sigma control charts but modify the control limitscalculated with correction from CEV weights for a fixed level ofcensoring. However, it assumes distribution parameters are known or theyhave to be estimated accurately from the complete type data (i.e. fromuncensored data). This requirement is not met in many practicalapplications. Therefore a practical alternative method is needed.

In statistical process control of reliability, many testing items areone side comparison, i.e. higher or lower measured results aredesirable. For example, in the iLD VBD reliability testing, the higherthe VBD values, the better of the processes which influence thisreliability testing. Therefore, monitoring its minimum VBD value from atest of certain sample size n shall constitute a good control chartcandidate. If its minimum VBD is in control, then the process is incontrol too. For order statistics, the minimum value from a test onsample size n has its own distribution different from the original(parent) distribution of the n samples (X(1), X(2), . . . X(i), . . .X(n), if they are not censored at all). If the parent distribution hasits pdf and cdf as fx(x) and Fx(x), then the pdf of the minimum X(1) hasthe following distribution.ƒ_(X) ₍₁₎ =n[1−F _(x)(x)]^(n-1)ƒ_(x)(x)

(Nancy R. Mann, etc, “Methods for Statistical Analysis of Reliabilityand Life Data, p. 91, 1974.)

The distribution of minimum will be off from normal distribution even ifthe parent distribution was normal distribution. Therefore, we need totransform the minimum's distribution to normal or close to normaldistribution before we use traditional method for control limitcalculation. Since the minimum value is an individual point, thetraditional method of control limit calculation after transformationuses the following formula:

Where MR is the moving range of successive observations.LCL= X−3[ MR/1.28]

The following example demonstrates how the minimum values are used forcontrol limit calculation.

A reliability test of VBD (voltage of break down) is needed to monitorits performance weekly. The sample size of a test is 26. The maximumramped up voltage for break down test is limited to 100 volts due toconcern of potential probe card burning on testing tools. The collecteddata show different percentage of censored observations. Some of testresults of one test have all censored observation, i.e. none of the 26dice failed during testing with 100V cut off scanned voltage.

First of all, outliers are screened out with Grubbs' method. We then useBox-Cox transformation technique to transfer the non-normal data intonormal or close to normal distribution. Then apply a traditional methodof control limit calculation for individual measurement to calculate thecontrol limits.

The FIG. 3 shows the normal quantile plot from the original VBD datawithout outlier screening and Box-Cox transformation. This diagram ismerely an example, which should not unduly limit the scope of the claimsherein. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives. You can see it is very muchoff from normal distribution. FIG. 4 is the normal quantile plot forbreakdown voltage after outlier screening and transformation accordingto an embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims herein.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

After two outliers are removed and Box-Cox transformed, its normalquantile plot in FIG. 4 shows that the data are around the straight lingwhich indicates normal distribution. FIG. 5 shows the histogram afteroutlier removing and transformation. This diagram is merely an example,which should not unduly limit the scope of the claims herein. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

The optimized Box-Cox transformation is Y=(X^2−1)/174.95 where X is theobservation before transformation and Y is after transformation.

For the control chart with one side Specification, only one side controllimit is needed. The low control limit (LCL) with the transformed datawas obtained using the above formulae. The calculated Avg X is 44.176and Avg MR is 7.506 with Box-Cox transformed data. The LCL fortransformed data is 26.59 and therefore the LCL for original VBD controlchart is 68.2 volts, calculated with the Box-Cox transformationformulae. Therefore the control chart with calculated control limit isshown in FIG. 6. This diagram is merely an example, which should notunduly limit the scope of the claims herein. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

The data shown in control chart include the censored data points.However the control limit calculation is based on only the minimum dataother than 100 volts. We divide the samples into two groups. These testswith one or more VBD data points less than 100 Volts are considered asthe first group and the other tests with all 100 Volts are considered assecond group. We only monitor the first group in control chart.

Conclusion: Appropriate transformation to normality with Minimumstatistic makes it feasible to calculate control limits for censoredmonitor data. Of course, there can be variations, modifications, andalternatives.

Depending upon the specific embodiment, the system is overseen andcontrolled by one or more computer systems, including a microprocessorand/controllers. In a preferred embodiment, the computer system orsystems include a common bus, oversees and performs operation andprocessing of information. The system also has a display, which can be acomputer display, coupled to the control system, which will be describedin more detail below. Of course, there can be other modifications,alternatives, and variations. Further details of the present system areprovided throughout the specification and more particularly below.

FIG. 7 is a simplified diagram of a computer system 900 that is used tooversee the method of FIG. 1 according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other modifications, alternatives, and variations.As shown, the computer system includes display device, display screen,cabinet, keyboard, scanner and mouse. Mouse and keyboard arerepresentative “user input devices.” Mouse includes buttons forselection of buttons on a graphical user interface device. Otherexamples of user input devices are a touch screen, light pen, trackball, data glove, microphone, and so forth.

The system is merely representative of but one type of system forembodying the present invention. It will be readily apparent to one ofordinary skill in the art that many system types and configurations aresuitable for use in conjunction with the present invention. In apreferred embodiment, computer system 900 includes a Pentium™ classbased computer, running Windows™ NT operating system by MicrosoftCorporation or Linux based systems from a variety of sources. However,the system is easily adapted to other operating systems andarchitectures by those of ordinary skill in the art without departingfrom the scope of the present invention. As noted, mouse can have one ormore buttons such as buttons. Cabinet houses familiar computercomponents such as disk drives, a processor, storage device, etc.Storage devices include, but are not limited to, disk drives, magnetictape, solid-state memory, flash memory, bubble memory, etc. Cabinet caninclude additional hardware such as input/output (I/O) interface cardsfor connecting computer system to external devices external storage,other computers or additional peripherals, which are further describedbelow.

FIG. 8 is a more detailed diagram of hardware elements in the computersystem according to an embodiment of the present invention. This diagramis merely an example, which should not unduly limit the scope of theclaims herein. One of ordinary skill in the art would recognize manyother modifications, alternatives, and variations. As shown, basicsubsystems are included in computer system 900. In specific embodiments,the subsystems are interconnected via a system bus 1385. Additionalsubsystems such as a printer 1384, keyboard 1388, fixed disk 1389,monitor 1386, which is coupled to display adapter 1392, and others areshown. Peripherals and input/output (I/O) devices, which couple to I/Ocontroller 1381, can be connected to the computer system by any numberof means known in the art, such as serial port 1387. For example, serialport 1387 can be used to connect the computer system to a modem 1391,which in turn connects to a wide area network such as the Internet, amouse input device, or a scanner. The interconnection via system busallows central processor 1383 to communicate with each subsystem and tocontrol the execution of instructions from system memory 1382 or thefixed disk 1389, as well as the exchange of information betweensubsystems. Other arrangements of subsystems and interconnections arereadily achievable by those of ordinary skill in the art. System memory,and the fixed disk are examples of tangible media for storage ofcomputer programs, other types of tangible media include floppy disks,removable hard disks, optical storage media such as CD-ROMS and barcodes, and semiconductor memories such as flash memory,read-only-memories (ROM), and battery backed memory.

Although the above has been illustrated in terms of specific hardwarefeatures, it would be recognized that many variations, alternatives, andmodifications can exist. For example, any of the hardware features canbe further combined, or even separated. The features can also beimplemented, in part, through software or a combination of hardware andsoftware. The hardware and software can be further integrated or lessintegrated depending upon the application. Further details of certainmethods according to the present invention can be found throughout thepresent specification and more particularly below.

The disclosures and the description herein are purely illustrative andare not to be limited with the above examples. A person skilled inreliability engineering and reliability statistics would be able toapply the method disclosed in the above embodiments to his/herparticular product, component or system in reliability testing. It isalso understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims.

1. A method for monitoring device characteristics of semiconductorintegrated circuits, the device characteristics including censored dataand uncensored data, the method comprising: determining a plurality ofminimum breakdown voltages numbered from 1 through N, respectively, fora plurality of lots numbered from 1 through N, each of the plurality ofminimum breakdown voltages being respectively indicative of theplurality of samples through order statistics, one or more of theplurality of samples including one or more uncensored data points andone or more censored data points; processing the minimum breakdownvoltages, respectively, for the plurality of lots, each of the minimumbreakdown voltages processed for the respective plurality of lots beingindicative of a population characteristic breakdown voltage numberedfrom 1 through N for the respective lot numbered from 1 through N;determining one or more anomalies based upon the processing of theminimum breakdown voltages, the one or more anomalies being associatedwith one or more processes associated with at least one of the lots; andproviding an output based upon the one or more anomaly.
 2. The method ofclaim 1 wherein the processing the plurality of minimum breakdownvoltages comprises a Box-Cox transform.
 3. The method of claim 1 whereinthe processing the plurality of minimum breakdown voltages comprises atransform process.
 4. The method of claim 1 wherein the plurality ofbreakdown voltages are associated with one or more interlayer dielectricmaterials for the respectively semiconductor devices.
 5. The method ofclaim 1 wherein the one or more censored data skews an average valueassociated with the plurality of samples for at least one of theplurality of lots.
 6. A method for monitoring device characteristics ofsemiconductor integrated circuits, the device characteristics includingcensored data and uncensored data, the method comprising: determining aplurality of average breakdown voltages numbered from 1 through N,respectively, for a plurality of production lots numbered from 1 throughN, where N, each of the plurality of production lots including aplurality of samples, the plurality of samples including one or morecensored data points and one or more uncensored data points; determininga plurality of minimum breakdown voltages numbered from 1 through N,respectively, for the plurality of lots numbered from 1 through N, eachof the plurality of minimum breakdown voltages being indicative of theplurality of samples, including the one or more uncensored data pointsand the one or more censored data points; processing the minimumbreakdown voltages for each of the plurality of lots; and determiningone or more anomalies based upon the processing of the minimum breakdownvoltages; and providing an output based upon the one or more anomaly. 7.The method of claim 6 wherein the plotting comprises processing theplurality of minimum breakdown voltages using a Box-Cox transform. 8.The method of claim 6 wherein the plotting comprises processing theplurality of minimum breakdown voltages using a transform process. 9.The method of claim 6 wherein the plurality of breakdown voltages areassociated with interlayer dielectric materials for the semiconductordevices.
 10. The method of claim 6 wherein the plurality of lotscorrespond respectively to a group of wafers for the manufacture ofintegrated circuits.
 11. The method of claim 6 wherein the wherein eachof the plurality of samples comprises an integrated circuit device. 12.The method of claim 1 wherein the minimum breakdown voltages correspondto breakdown voltages associated with respective transistor devices. 13.The method of claim 6 wherein the one or more anomalies correspond toone or more outlier points.
 14. The method of claim 6 wherein the outputcomprises a plot.
 15. The method of claim 6 wherein the processing ofthe minimum breakdown voltages comprises comparing the minimum breakdownvoltages against one or more control values.
 16. The method of claim 6wherein the determining of the minimum breakdown voltages comprisesmeasuring breakdown voltage values from the respective plurality ofsamples.